Rest/activity phase-responsive rate controlled cardiac pacer

ABSTRACT

A rate controlled cardiac pacemaker stimulates the heart of a patient at a stimulation rate and includes a stress sensor for sensing actual physical stress of the patient. The device includes control logic for adapting the stimulation rate of the pacemaker to the actual physical stress of the patient and determining circuitry for determining the activity of the patient. The determining circuitry includes an activity sensor for sensing the activity state of the patient and a rate control circuit having an input connected with an output of the activity sensor. A timer generates a signal based on a 24-hour rhythm indicating a rest phase or an activity phase, and is synchronized by the activity sensor in a 24-hour rhythm with patient activity and rest phases that occur in continuous alternation. The activity sensor, in the case of the absence of activity or low activity of the patient over a long period of time, or in the case of activity which exceeds a predetermined level, respectively, emits an output signal to an input of the timer for synchronizing the timer. The timer generates the signal based on the 24-hour rhythm indicating a rest phase or an activity phase, respectively, independently of the signal from the activity sensor, with a change between an activity and a rest phase, on the average, following a change of the output signals of the activity sensor picked up during a preceding time period.

This application is a continuation of application Ser. No. 07/859,448,filed May 29, 1992, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to a cardiac pacemaker.

The publication "Biomedizinische Technik (Biomedical Technology), 34(1989) pages 191-196" discloses a cardiac pacemaker developed forrate-adaptive single and dual chamber stimulation whose rate control iseffected on the basis of measuring the low frequency oscillationspectrum as signals characterizing the patient's activity. For thispurpose, the signals are limited in level, are amplified in a frequencyselective manner and their spectral component is evaluated for thephysical stress.

With the aid of a piezoelectric transducer integrated in the pacemakerhousing and with a subsequently connected signal processing circuit, themotion values generated by physical stress can be detected and thepacemaker can be optimally physiologically adapted to the patient'sgiven stress state.

However, the signal processing unit connected to the output of thepiezoelectric transducer that is integrated in the pacemaker housingrequires additional current beyond that required by the other circuitcomponents of the pacemaker independent of the patient's biorhythm.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a cardiac pacemaker whichadapts itself to repeated activities of the patient and, in particular,also reduces the energy requirement of the pacemaker during predictablecycles of reduced physical activity of the patient.

The invention is based on the realization that the daily rhythm of apatient is usually cyclical and thus it becomes possible, particularlywith reduced activity, to also adapt particularly the activity of thepacemaker to these cycles so that the components for detecting andadapting the heart rate to this activity can be operated with reducedenergy, since changes in the heart rate need not be initiated or onlyvery rarely.

The detection of the patient's activity can here be effected either by asensor that is separate from the sensor for determining physical stressfor controlling the heart rate as, for example, by a position sensor fordetermining the patient's position in space (recumbent or standing) orby the sensor for physical stress itself In that case, the averagefrequency of the determination of the momentary stress is preferablyraised with the stress so that the adaptation is slower during restphases.

The component for changing the heart rate may possibly be switched offcompletely during a rest phase. If the heart rate is determined by wayof a sensor for determining the performance capability of the patient,which sensor is separated from the activity sensor, this sensor may alsobe switched off during a rest phase. In the other case, regulartemporary switching in at relatively great time intervals is sufficient.

To determine the rest phase of a patient, a circuit is utilized whichdetermines the periodicity of the regular rest phases of the patient andderives a control signal therefrom that causes the energy consumption ofthe component influencing the pacemaker stimulation rate to be reduced.If necessary, the repetition rate of the temporary switching in of themeans for detecting activity may then also be reduced during the regularrest phases.

Other modifications and advantageous features of the invention aredefined, the description below and in the drawing.

In order to prevent the relatively current intensive evaluation programof the subsequently connected signal processing circuit of an activitysensor system from being activated without interruption, a circuit ispreferably employed which detects the periodic rhythm of the patient'sactivity and rest phases and switches the energy supply of the circuitfor the activity dependent control of the heart rate to its highestvalue only during the activity phases. Preferably, this results in anadaptation to a day/night rhythm (activity and rest phases) for thepatient with respect to the connection of the activity sensor systemduring the patient's day phase and its disconnection during the nightphase, respectively.

If a single activity sensor system is employed for controlling thereduction of the energy supply as well as the rate control, it isfavorable to increase the frequency of activation of the circuit for thedetection of activity (and thus the energy consumption) in times ofincreased activity (activity phase) or to reduce its response level,respectively, while in times of lower activity (rest phase) the numberof activations of the circuit per unit time is reduced (and thus alsothe energy consumption). The cyclic control as a function of the regularactivity and rest phases of a patient in a 24-hour rhythm utilizes this"activity dependent activity" control of the activity detection circuitonly during the patient's averaged rest phases.

The solution according to the invention ensures in an advantageousmanner that the circuit means for detecting activity are supplied withincreased energy only if the patient is in an activity phase where rapidadaptation of the heart stimulation rate to changes in stress isnecessary. Individual movements, as they occur, for example, during thesleep phase, will thus not cause the cardiac pacemaker to be switched toan increased stimulation rate.

A solution is proposed as advantageous which adapts itself to thechanging day/night rhythms of the patient as they occur, for example,upon a change from summer to winter or during air travel connected withdifferent time zones as well as to a day/night ratio other than one. Forthis purpose, a phase sequential system is employed in particular suchas, for example, a PLL [phase locked loop] system with its adaptabilityto the frequency and phase position of changing reference signals. Thereference phase position here is, for example, the transition from theactivity to the rest phase, with the length of the rest phase then beingretained in an integrating memory for average time values.

In another advantageous solution, an event counter is employed whichretains signals characteristic of the activity phases in the momentarilyaddressed memory location of an average value memory that is cyclicallyaddressable in a 24-hour rhythm. The number of activity events pickedup, on the average, in earlier 24-hour cycles is read outsimultaneously. Those memory locations which, on the average, store anumber of activity events that exceeds a predetermined minimum value,when read out at a momentary point in time, then generate a signalindicating an activity state.

In this way, the pacemaker is given a sort of "internal clock" whichenables it to adapt its own activity to that of the patient. Thus, inparticular, the repetition rate of measurements or calculations that areperformed only at time intervals in order to save energy is furtherreduced during the patient's rest phase.

In particular, the operation of a circuit for adapting the number ofmeasurements or calculations performed by the processor to the momentaryactivity of the patient, can be limited to the patient's rest phases. Inthis way, the heart rate is adapted relatively quickly to physicalstress during the regular activity phase (that is, usually during theday) while during the night phase, during which changes in stress aregenerally not expected, the adaptation is slower. This, however,corresponds entirely to the behavior of the patient himself who in sucha case will also react rather "sleepily". Only if the patient developsan activity phase regularly at certain (initially unusual) times, willthe system adapt itself to this regularity due to its time adaptability(learning capability).

To detect the day/night rhythm and also other arbitrary changes betweenactivity and rest phases for the patient, a sensor system may beemployed, in particular, which monitors the patient's activities andswitches on the heart rate control upon detection.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantageous features of the invention will be described in greaterdetail below together with a description of the preferred embodiment ofthe invention and reference to the drawing figures in which:

FIG. 1 is a diagram of the time sequence of a patient's activity in a24-hour rhythm;

FIG. 2 depicts an evaluation circuit as an embodiment of the invention;and

FIG. 3 is a block circuit diagram for a rate-controlled cardiacpacemaker equipped with the measures according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a diagram serves to initially illustrate the change betweenthe various activity phases of the patient. The figure shows theday/night activity of a patient with t1 as the active period and t2 asthe rest period. The beginning of the active period t1 may change due toexternal influences such as summer/winter or different times zones. Thepulsing ratio between day and night is also subject to changes. Thesechanges are detected by the so-called "internal clock" of a person.

FIG. 2 depicts an evaluation circuit for the changes in day/nightactivity which derives therefrom a control signal for switching off arate control system 12 in order to conserve energy. The activitycriterion may be derived from an activity sensor 10 in rate controlsystem 12. The rate control system may be any system that causes thepacemaker rate to follow the patient's momentary stress, for example, asensor for detecting the systolic intervals, an activity evaluationsystem or a system for converting the patient's determined respirationrate to a value representative of the patient's momentary stress. Theoutput of rate control system 12 is connected to a cardiac pacemaker 14which performs an optimum pacemaker function in dependence on thepatient's stress.

From the output signal of the activity sensor system 12, a decoder 13obtains a reference frequency for a PLL circuit. Decoder 13 generates avoltage curve for the reference frequency which corresponds to FIG. 1.

A switch 19 can also be employed to derive the reference frequency froma position sensor 11 or a motion sensor. The reference frequency is thenfed to a phase comparison circuit 15. As the second frequency, phasecomparison circuit 15 receives the output signal of a voltage controlledoscillator 17 by way of a divider 18. The output signal of phasecomparison circuit 15 is conducted through a lowpass filter 16 to thevoltage controlled oscillator 17. The output signal of voltagecontrolled oscillator 17 is employed, if required by way of a divider(not shown), as the control signal for a controllable switch 20 whichduring the day phase connects the activity sensor system 12 to thesupply voltage V and during the night phase disconnects this supply.

Instead of being disconnected completely, switch 20 may also be switchedin periodically at a reduced repetition rate in order to save energy inthis way.

Since the pulsing ratio between day and night may be different in apatient, the invention additionally provides an integrating memory 21which generates a voltage value that is proportional to the pulsingratio. This voltage value is preferably employed to adapt the pulsingratio of the frequency of the voltage controlled oscillator 17 to thepulsing ratio between day and night.

A PLL circuit is employed in this connection which includes a voltagecontrolled oscillator 17 operating at a low frequency. In this way, theperiodicity of the moment of awakening in the morning is predictedcorrectly with great probability. The PLL system detects the phaseposition of the daily rise or drop in physical activity at the end orbeginning of the daily rest phase and sets the pacemaker activityaccordingly. This requires merely a matching of phase positions sincethe 24-hour repetition rate is determined by the revolution of the eartharound the sun--and thus the natural day/night rhythm--and is fixed forall patients. However, phase shifts to be compensated result, forexample, from air travel (jet lag). The above-described circuit need notbe set like a clock since its 24-hour frequency also adapts itself tothe daily rhythm of the patient so that no highly accurate timer isrequired.

In another embodiment of the invention, component groups 15 to 18 and 21are replaced by a memory that is cyclically addressed in a 24-hourrhythm and has a memory location for each one of a number of timeintervals. The momentary activity events are added to the stored averagevalue for the momentarily addressed time interval for averaging, whilethe memory contents in the memory location associated with the previousreaching of the respective time interval is read out and the outputsignal actuates a switch V, that is, the switch closes for a storedvalue indicative of increased activity or closes at a higher repetitionrate while for a memory content belonging to a lower activity, switch 20is opened or closed at a lower frequency, respectively, in order toprevent the energy supply of the pacemaker from being exhaustedprematurely.

In this way, the pacemaker detects the patient's biorhythm and adaptsthe stimulation behavior accordingly like an "internal clock" withoutrequiring additional measuring value pickups for this purpose.

FIG. 3 is a block circuit diagram for a rate controlled cardiacpacemaker 100. This cardiac pacemaker stimulates by way of controlstages 101 and 102, respectively, and output stages 103 and 104,respectively, the atrium 105 and the ventricle 106, respectively of theheart 107 by means of appropriate electrodes that ar connected to theoutput circuits. These electrodes also pick up from the heart signalscharacteristic for the action of the heart itself and feed these to aninput amplifier circuit 108 and 111, respectively. These signals areprocessed by way of subsequently connected detection stages 110 and 109,respectively--in each case separately for atrium and ventricle.

These data are conducted to a processor system 113 by way of logiccontrol unit 112. This processor system 113 is connected with the logiccontrol unit M2 by way of a data bus, a control bus and an additionalbus so that the digitized signals picked up in the atrium and ventriclecan be read out from the processor unit and additional digital controlsignals can be put output to generate stimulation pulses in the atriumand ventricle. Logic control unit 112 has the same relationship toprocessor unit 113 as a control unit for a peripheral unit, for example,an interface equipped with an A/D-D/A converter for picking up andputting out external analog signals, often also called an input-outputunit

Also connected with the logic control unit 112 is a bidirectionaltransmission channel 114, 115 which is able to transmit by means of aninductance 116 signals picked up from the heart to outside of thepatient, and also control signals for programming the cardiac pacemakerfrom outside the patient to the pacemaker. Further provided is a battery117 for supplying the entire pacemaker, a quartz crystal 118 and aresonant circuit 119 as timer or reserve timer, respectively.

A signal pickup unit 120 serves to determine a physiological parameterwhich constitutes a measure for physical stress within the patient'sbody. In the illustrated embodiment this unit is a pulse generator forputting out measurement pulses (current pulses i) in the right ventriclefrom which can be determined by way of the electrode disposed in theventricle the momentary electrical resistance within the ventricle atpredetermined times. Circuit 120 further includes a line fordisconnecting this module so that the emission of current pulses, inparticular, is prevented. Logic control unit 112 is able to disconnectblock 120 by means of an "off" signal. This disconnection is effected byprocessor unit 113.

Finally, a rest phase switch 121 is shown which detects phases ofphysical activity and rest in the patient. The rest phase switch isconnected with processor system 112 for direct data exchange. Rest phaseswitch 121 includes the PLL unit described above which includes aposition or motion sensor. This unit is configured in such a way thatthe output signal of the position or motion sensor is linked by means ofAND gates with the output signal of the unit detecting the patient'sactivity rhythm. Sporadic movements or changes in the patient's positionare detected as the beginning of an activity phase only if the cycle ofthe expected activity phase has also begun so that the cardiac pacemakeris prevented from beginning an operating mode associated with thepatient's active phase (daytime program) already upon individual changesin position, for example during sleep. Only if the system detects moreintensive activity of the patient outside of a phase in which a restphase would actually be expected, will the pacemaker also switch to itsprogram associated with the active phase. In this program, for example,block 121 is activated by suppression of the "off" signal so that thestimulation rate is change adequately for physical stress. Since thiscircuit consumes a considerable amount of battery energy because of itsregularly generated current pulses, its use can be dispensed with duringthe patient's rest phases. The arrangement according to the inventionensures that activation takes place only if movements or changes in thepatient's position actually belong to a longer-lasting activity.

By means of a reed switch 122 and an external magnet, processor unit 113can be put into a predetermined state in which stimulation is effectedpreferably at a fixed rate.

The invention is not limited in its embodiments to the above-describedpreferred embodiment. Rather, a number of variations are conceivablewhich take advantage of the described solution even for basicallydifferent configurations.

What is claimed is:
 1. A rate controlled cardiac pacemakercomprising:stimulating means for stimulating the heart of a patient at astimulation rate; a stress sensor for sensing actual physical stress ofthe patient; logic control means coupled with the stimulating means andthe stress sensor, for adapting the stimulation rate to the actualphysical stress of the patient; determining means coupled with the logiccontrol means, for determining the activity of the patient, saiddetermining means including an activity sensor for sensing the activitystate of the patient, said activity sensor having an output, and a ratecontrol circuit having an input connected with the output of theactivity sensor; and timer means coupled with the determining means, forgenerating a timer output signal based on a 24-hour rhythm indicating arest phase or an activity phase, synchronized by said activity sensor ina 24-hour rhythm with patient activity and rest phases that occur incontinuous alternation, said activity sensor, in the case of the absenceof activity or low activity of the patient over a long period of time,or in the case of activity which exceeds a predetermined level,respectively, emitting an output signal to an input of said timer meansfor synchronizing the timer means, said timer means generating the timeroutput signal based on the 24-hour rhythm indicating a rest phase or anactivity phase, respectively, independently of the signal from saidactivity sensor, with a change between an activity and a rest phase, onthe average, following a change of the output signals of the activitysensor picked up during a preceding time period.
 2. A cardiac pacemakeraccording to claim 1, wherein said determining means comprises:aphase-locked-loop circuit including a voltage controlled oscillatorhaving at least one input, wherein the output signal of the activitysensor provides a reference frequency for the at least one input of thevoltage controlled oscillator of the phase-locked-loop circuit.
 3. Acardiac pacemaker according to claim 2, wherein the phase-locked-loopcircuit voltage controlled oscillator operates at a low frequency thatlies in the frequency range of an activity/rest rhythm.
 4. A cardiacpacemaker according to claim 3, further comprising a controllable switchconnected between said rate control circuit and an external supplyvoltage,wherein the voltage controlled oscillator generates a controlsignal for controlling the controllable switch, the controllable switchconnecting and disconnecting said rate control circuit to the externalsupply voltage.
 5. A cardiac pacemaker according to claim 4, furthercomprising an additional sensor and an AND gate means connected toreceive the timer output signal from the timer means and an outputsignal of the additional sensor,wherein the timer output signal of thetimer means is linked in a logical AND relationship by the AND gatemeans with the output signal of said additional sensor so that a signalcharacteristic of the activity phase of the patient is output only if asignal characterizing a change in at least one of motion and position ofthe patient from said additional sensor coincides with the timer outputsignal from the timer means characterizing an activity phase.
 6. Acardiac pacemaker according to claim 1, further comprising evaluationmeans for receiving input from the activity sensor and input from thetimer means, and having an evaluation threshold to be exceeded, forevaluation of patient activity signals from said activity sensor, saidevaluation threshold being raised during a patient rest phase.
 7. Acardiac pacemaker according to claim 1, further comprising switchingmeans, connected to said timer means, for periodically switching on saidlogic control means, wherein during a patient rest phase, said logiccontrol means is switched on a reduced number of times by said switchingmeans.
 8. A cardiac pacemaker according to claim 7, wherein during therest phase, said logic control means is switched on by said switchingmeans a reduced number of times;whenever said determining means fordetermining the patient's activity outputs an output signal, whenever apredetermined minimum activity is not reached for a predeterminedminimum time period as determined by said determining means, or wheneverthe sum of these time periods exceeds a predetermined value within apredetermined time interval.
 9. A cardiac pacemaker according to claim1, wherein said determining means comprises:an event memory havingaddressable memory locations therein, wherein the output signal of theactivity sensor provides an event signal to be retained in said memorylocations of said event memory means, and wherein said event memorymeans periodically, in a 24-hour cycle, addresses said memory locationstherein for said event signal and provides the output signal to saidvoltage controlled oscillator, thereby performing time dependent controlthat can be synchronized in a 24-hour rhythm.
 10. A cardiac pacemakeraccording to claim 9, wherein the phase-locked-loop circuit voltagecontrolled oscillator operates at a low frequency that lies in thefrequency range of an activity/rest rhythm.
 11. A cardiac pacemakeraccording to claim 10, further comprising a controllable switchconnected between said rate control circuit and an external supplyvoltage,wherein the voltage controlled oscillator generates a controlsignal for controlling the controllable switch, the controllable switchconnecting and disconnecting said rate control circuit to the externalsupply voltage.
 12. A cardiac pacemaker according to claim 11, whereinsaid event memory means comprises an integrating memory which generatesa voltage value that is proportional to a pulsing ratio of theactivity/rest rhythm, the voltage value generated by the integratingmemory adapting a pulsing ratio of the frequency of the voltagecontrolled oscillator to a day to night ratio.
 13. A cardiac pacemakeraccording to claim 12, further comprising an additional sensor and anAND gate means connected to receive the timer output signal from thetimer means and an output signal of the additional sensor,wherein thetimer output signal of the timer means is linked in a logical ANDrelationship by the AND gate means with the output signal of saidadditional sensor so that a signal characteristic of the activity phaseof the patient is output only if a signal characterizing a change in atleast one of motion and position of the patient from said additionalsensor coincides with the timer output signal from the timer meanscharacterizing an activity phase.
 14. An evaluation circuit fordetecting changes in a patient day/night activity and producing acontrol signal for switching off a rate control system for a cardiacpacemaker, the evaluation circuit comprising:an activity sensor forproviding signals to the rate control system; a decoder operativelycoupled to receive signals from the rate control system and produce areference frequency signal output; a phase locked loop circuit forreceiving the reference frequency signal from the decoder and producinga control signal; and a controllable switch coupled between the ratecontrol system and a power source, the controllable switch furthercoupled to said phase locked loop circuit and being controlled by thecontrol signal from said phase locked loop circuit.
 15. The evaluationcircuit according to claim 14, further comprising an integrating memoryfor generating a voltage that is proportional to a patient pulsing ratiobetween day and night and providing the voltage to said phase lockedloop circuit to adapt a frequency of said phase locked loop circuitbetween day and night values.
 16. The evaluation circuit according toclaim 15, further comprising an additional sensor, the additional sensorbeing one of a position and a motion sensor, and a selector switchwherein the selector switch is coupled to the rate control system, thedecoder and the additional sensor, for switching signals to the detectorfrom either the rate control system or the additional sensor.
 17. Theevaluation circuit according to claim 14, wherein the phase locked loopcircuit comprises:a phase comparator for receiving a first and a secondinput signal, the first input signal being the reference frequencysignal output by the decoder and providing a comparison signal output; alow pass filter for receiving the comparison signal from the phasecomparator and producing a filtered output signal; a voltage controlledoscillator for receiving the filtered output signal from the low passfilter and producing the control signal; and a divider for receiving thecontrol signal from the voltage controlled oscillator and providing thesecond input signal to the phase comparator.
 18. A rate controlledcardiac pacemaker comprising:controllable output means for stimulating apatient heart with electrical signals; control means for controlling theoutput means; amplifier means for receiving and amplifying signals fromthe patient heart; detection means for receiving the amplified signalsfrom the amplifying means and providing detection signals; rest phaseswitch means for detecting phases of physical activity and rest of apatient and producing activity signals; and logic control means forreceiving the activity signals from said rest phase switch means and thedetection signals from said detection means, and providing controlsignals to said control means.